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Human locomotion is a phenomenon of the most extraordinary complexity in which so great are the multitude of individual motions occurring simultaneously in the three planes of space that analysis is difficult without some unifying principle. The adoption of the concept that fundamentally locomotion is the translation of the center of gravity through space along a pathway requiring the least expenditure of energy supplies the necessary unifying principle which permits of qualitative analysis in terms of the essential determinants of gait. The six major determinants are pelvic rotation, pelvic tilt, knee and hip flexion, knee and ankle interaction, and lateral pelvic displacement. The serial observations of irregularities in these determinants provides insight into individual variation and a dynamic assessment of pathological gait. Pathological gait may be viewed as an attempt to preserve as low a level of energy consumption as possible by exaggerations of the motions at unaffected levels. Compensation is reasonably effective with the loss of one determinant of which that at the knee is the most costly. Loss of two determinants makes effective compensation impossible and the cost of locomotion in terms of energy is increased threefold with an inevitable drain upon the body economy.